1. Field of the Invention
Our invention is concerned with polyimide oligomerdinitride dioxide copolymers and methods for the preparation thereof. The subject matter of Group Art Class 260 can be characterized as generally related.
2. Description of the Prior Art
The patent literature has fully disclosed the preparation of polyimide oligomers which, when fully cured, are relatively insoluble and intractable materials. When there materials are used as laminating resins, adhesives, etc., they are applied in the form of polyamic acid or ester precursors and then cured. This cure step produces water or alcohol as byproducts, that are volatile at the elevated temperatures required to accomplish complete curing which result in the formation of large amounts of undesirable voids in the final composition assembly.
The advantage of our invention over the current state of the art lies in the ability of our polyimides to cure without the evolution of copious quantities of volatile byproducts such as are seen when conventional polyamic acid based polyimide precursors are cured. Our copolymers thus have very low void contents in their cured or fabricated states. Low void characteristics are not unique for polymers such as the epoxies which are useful to approximately 300.degree.-400.degree. F. However, this characteristic is unique for high temperature polymers such as the block copolymers of our invention which are stable to approximately 600.degree. F.
We have discovered that cyano substituted polyimide oligomers, and acetylene substituted polyimide oligomers will copolymerize by an addition polymerization process. These oligomers are new to the art as disclosed by U.S. Pat. applications Ser. Nos. 347,501 and 347,535. Conventional polyimide oligomers form copolymers by condensation polymerization processes generally and thus do not exhibit the low void characteristics of the copolymers of our invention when fully cured.
The literature discloses bis maleimide type polyimides that have been marketed which cure via addition polymerization processes. However, it should be noted that with these polyimides cyclopentadiene is liberated as a byproduct during the initial period prior to cure. This characteristic, therefore, tends to limit their usefulness in applications where a non-outgassing polymer is required. Furthermore, these bis maleimide type polyimides lose a larger proportion of their cyclopentadiene when they are used as thin coatings than they do when used in thick section laminates or molding compounds. Thus, their compositions differ from application to application yielding products whose properties are not consistent within a given design.
Dinitrile dioxide reactants have been used to form copolymers with difunctional comonomers having acetylenic and ethylenic unsaturation (Frazz, U.S. Pat. No. 3,213,068). These copolymers are characterized by the fact that they contain an isoxazoline ring. ##STR1## and/or an isoxazole ring in the backbone ##STR2## or linear chain of the polymer, and are prepared from nonpolymmeric reactants. It can be concluded that this patent teaches that ethynyl substituted aromatic oligomers would tend to yield products useful primarily for compression molding.